This invention relates generally to high speed analog-to-digital converters and particularly relates to such converters utilizing Gunn effect devices and field effect transistors.
The analog-to-digital converter is a key element in digital signal processing. In recent years signal processing techniques have gone to signals of higher center frequency and wider bandwidth because this will yield a high performance system with relatively low cost. It will, therefore, be evident that the speed, that is the sampling rate per second and the accuracy of the converter limits this type of application.
The bandwidth of the signal to be converted into digital signals is limited to less than half the sampling rate. Therefore, the clock pulse rate is also of importance. The number of levels into which the amplitude of the analog signal is converted limits the amount of information available and imposes a signal-to-noise ratio constraint. This is due to the fact that the number of levels into which the analog signal is converted is limited rather than being infinite. Thus, an error may be caused because a signal level may not be quite at the lower or upper limit. Therefore, the analog-to-digital converter limits the amount of information available for subsequent digital processing.
Typical state-of-the-art converters provide sample rates of 400 megasamples (MS) per second and with 5 bit conversion levels or resolution. It is, therefore, desirable to provide higher sample rates which present converters cannot provide.
Analog-to-digital converters could either be used in parallel or in a series or cascade configuration. The series configuration is preferred because it requires less equipment and hence is less costly. However, for high accuracy both the signal amplitude and the time delay between successive sampling periods must be controlled with great precision. Generally delays of a few nanoseconds between successive stages and successive sampling periods are required. Such relatively large time delays cannot be obtained on an integrated circuit chip. Therefore, it is necessary to transfer the signal from the integrated circuit chip to a transmission line and back to the same or a different chip. It has been a most difficult problem to transfer without distortion a wide bandwidth signal on or off the chip.
Since the maximum clock rate available with silicon devices produces delays of several nanoseconds, it has not been possible to provide a high speed analog-to-digital converter with a series configuration. Therefore, converters with parallel configurations had to be used. They require a larger number of components and use more power and hence are more expensive to build or operate. Thus in part the performance of the analog-to-digital converter is limited by the cut-off frequency of silicon integrated circuit transistors. Hence the clock rate which determines the sample rate can be no greater than 1/5 of the transistor cut-off frequency and this amounts to a sample rate of 400 megasamples per second as previously referred to.
Accordingly, in accordance with the present invention use is made of Gunn effect devices sometimes known as transferred electron devices as well as field effect transistors. The latter make use of gallium arsenide (GaAs). This will permit a substantial increase in speed.
It is accordingly an object of the present invention to provide a parallel or serial analog-to-digital converter having substantially increased speed and a higher sample rate.
A further object of the present invention is to provide an analog-to-digital converter making use of transferred electron devices and field effect transistors.
Another object of the present invention is to provide an analog-to-digital converter which can be realized as an integrated circuit chip with vastly increased speed or sampling rate.
Still a further object of the present invention is to provide an analog-to-digital converter which can be realized in series configuration and which has time delays on the order of 100 picoseconds.
The Gunn effect has been discovered in 1963 by J. B. Gunn who found that coherent microwave oscillations may be generated in bulk gallium arsenide semiconductor material. The physics of such Gunn effect devices has been explained in a book by S. M. Sze, "Physics of Semiconductor Devices" published by John Wiley and Sons 1969 (see particularly pages 731 - 784).
Specifically, a Gunn triode will amplify an input signal and operates in the microwave region. The device itself is extremely small, that is on the order of 10 - 20 microns long. Even though the effect is a bulk effect, the thickness of the effective layer may be on the order of 1/10 of its length, that is 1 - 2 microns thick. Such devices are characterized not only by their small size, but by appreciable isolation between input and output terminals.